A various type of tire pressure monitoring systems has been proposed. One example of the kind is shown in U.S. Pat. No. 5,602,524. The tire pressure monitoring system includes sensor units and a monitoring unit. Each sensor unit is constructed of a sensor for sensing a tire pressure and a transmitter for sending a signal indicative of a sensed tire pressure, and mounted to a tire. The monitoring unit, which is mounted in a vehicle, receives the signal from the sensor unit and issues a warning when the tire pressure is equal to or lower than a predetermined level.
In this system, an identification (ID) code is uniquely assigned to each sensor unit for determining whether the received signal is sent from the sensor unit. The ID code is sent to the monitoring unit with the sensed tire pressure. The monitoring unit stores the ID code of each sensor unit registered in advance. It determines whether the sensed tire pressure is within a proper range only when the ID code contained in the signal matches the stored ID code. By using the ID code, the reliability of the tire pressure monitoring is improved. For example, improper tire pressure monitoring performed based on signals from sensor units mounted to tires of other vehicle is less likely to occur.
When the ID code is registered to the monitoring unit, the mode of the monitoring unit is switched from a monitoring mode (regular mode) to a register mode by a predetermined switching operation. Then, the tire pressure is rapidly reduced so that the signal containing the ID code is forcefully sent from the sensor unit. The ID code is registered to the monitoring unit when it is in the register mode and has received the signal from the sensor unit.
To register the ID code, a switch for switching the mode of the monitoring unit to the register mode is required. When switching to the register mode or returning to the regular mode is not properly performed, the ID code may not be properly registered in the monitoring unit. As a result, the tire pressure monitoring process may not be properly performed.